1. Field of the Invention
The present invention relates to an apparatus for the reproduction of video signals on a monitor, having a circuit connected to the monitor for the transformation of grayscale values of a video signal.
2. Description of the Prior Art
Reproduction systems of the type described above can be used, for example, in x-ray diagnostic arrangements that have an image transformer for the conversion of an x-ray image into an electric signal sequence, such as a video signal. Such an image transformer can be, for example, a television camera or a storage foil.
These analog video signals are in general converted in an analog/digital converter (A/D-converter) into digital values so that the x-ray image is resolved into different image points (pixels), to which different digital grayscale values are allocated. In a reproduction device the digital video signal can be processed and, through a digital/analog converter (D/A-converter) be displayed on a monitor as a visible picture.
In order to obtain a contrast-rich image on the monitor over the entire image range that contains diagnostically relevant information, the D/A converter should be offered, in the image region of interest, the entire range of the grayscale values, i.e., a full dynamic should be utilized. This condition, however, is normally not fulfilled, since, for example, the image converter is controlled independently of the grayscale values, so that the dynamic range of the A/D converter cannot be optimally utilized.
Further, in x-ray diagnostics lead plates are ordinarily used to blank outer regions of the image, so that the darkest images regions lie outside the image region of interest. In x-ray exposures, for example, of the skull or of the extremities blooming can occur in the image region of interest because the completely dark image regions were used as the basis for the low end of the grayscale allocation. The results in image areas of maximal brightness which carry no diagnostic information data.
In order to achieve an optimum grayscale value image presentation, it is known to amplify or "window" all the pixels of the video signal, before they are conducted to the D/A converter, in such a way that the grayscale value zone of interest is spread onto the full brightness range of the image monitor. Grayscale values of the video signal that lie outside of this window, however, are still represented only as black or white on the image monitor.
In U.S. Pat. No. 4,827,492 a window system is described in which two operating elements are provided for the adjustment of the window. In this known system, one operating element sets the window width and another sets the window middle, or the upper or lower window boundary. What is disadvantageous here is that a complicated adjustment has to be undertaken, in which the two operating elements are alternatingly manipulated for an optimum of the image. Reproducible picture results, however, are achievable only with difficulty. Reproducible results, however, are required, especially for a hardcopy documentation for which the image quality must be kept constant, for example, on a film.